The present invention relates to rotary drum filters used in the pulp and papermaking industry to form a mat of wood pulp and separate the mat from its filtrate. The invention relates especially to such filters of the pipe machine type in which there is a suction control valve located centrally of one end of the drum filter for controlling the application of suction to the filter surface and the drawing of filtrate from the mat and drum. More specifically, the invention relates to an improved stationary suction valve and discharge elbow assembly for such a filter.
A drum filter of the pipe machine type comprises a rotary drum partially submerged in a tank of pulp slurry. Typically, the drum has axially extending filtrate channels spaced about its peripheral surface defined by ribs and covered by a filter screen. The surface filtrate channels communicate with an external source of subatmospheric pressure, or suction, through drainage pipes, an annular valve housing mounted centrally within the discharge end of the drum, a stationary valve member seated within the valve housing and extending therefrom, and a discharge elbow connected to the outlet end of the valve member. The stationary valve member has a valve closure segment which blocks off a section of multiple discharge ports in the valve housing to control the application of subatmospheric pressure to the connected drainage pipes and surface filtrate channels. As the drum rotates about its axis with suction applied, the screen rotates through the pulp slurry and collects a wet mat or "cake" of fibres from the slurry. As the screen emerges from the tank, filtrate is drawn from the mat through the screen and filtrate channels into the drainage pipes, and from there through the valve housing and into the valve member and discharge elbow to removethe filtrate from the drum. During rotation of the drum, the stationary valve closure segment cuts off suction to the filtrate channels in a segment of the drum surface, thereby enabling removal of the pulp mat from the screen surface, usually with the aid of a doctor device.
Rotary drum filters of the described pipe machine type are shown, for example, in U.S. Pat. No. 3,363,774 to Luthi and U.S. Pat. No. 4,383,877 to LaValley. Typically, the stationary valve member for such a drum filter, partially shown in FIG. 7 of the Luthi patent, has a cylindrical valve plug at its inlet end which is seated within the valve housing. A conically tapered valve closure segment extends rearwardly from the plug to cover a portion of the valve housing outlet ports to block them from the source of suction. A large central shaft extends rearwardly from the plug and outwardly beyond the discharge end of the drum, connecting with an annular flange and sealing rim at the outlet end of the valve member, such outlet end portion not being shown in the Luthi patent. Usually, a generally cylindrical drum trunnion connected to the drum surrounds the shaft portion of the valve member to define a portion of the filtrate discharge passage leading from the drum. The outlet end of the valve member joins an inlet end of the discharge elbow. The outlet end of the elbow, in turn, connects to a suction pipe leading to the source of subatmospheric pressure. The typical discharge elbow, shown at 30 and 31 in FIG. 2 of the Luthi patent, defines a flow passage having a sharp right angle turn. This abrupt change of direction at the elbow corner creates substantial head loss at this point, reducing substantially the suction available at the filtrate channels.
The design of the described valve member creates additional head loss problems. Because of the large central shaft of the valve member, there is an abrupt increase in the cross sectional area of the discharge flow passage at the intersection of the valve member and discharge elbow, which creates additional head loss. Many valve members also include internal reinforcing struts or webs extending between the shaft and valve segment and between the shaft and sealing rim. These create obstructions and therefore turbulence in the flow passage, and abrupt changes in cross sectional area of the passage, further contributing to the head loss problem at the valve-discharge elbow assembly.
Excessive head loss between the source of suction and the filtrate channels at the surface of the drum is a contributing cause to inadequate dewatering of the pulp mat, poor drainage of the filtrate channels, and undesirable rewetting of the mat as the drum rotates downward to its mat removal position. Accordingly, there is a need for an improved valve-discharge elbow assembly which will reduce head loss in these sections of the filtrate discharge flow passage from the drum.
The principal objective of the present invention, therefore, is to reduce head loss between the source of subatmospheric pressure and the filtrate channels of a rotary drum filter of the pipe machine type through elimination of abrupt changes in direction and cross sectional area of the discharge flow passage, and through elimination of obstructions in such flow passage, all in the valve and discharge elbow portions of the filter drainage system.